Low distortion, DSP based high gain servo controlled woofer controller. - Page 19 - diyAudio
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Old 9th July 2013, 01:54 AM   #181
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Sounds good. Thanks. Looking forward to further development and testing. Your approach is certainly fertile for the future.

But not many people are aware of the daunting challenges to testing DSP programs. Not easy to test for DSP software performance let alone input-output performance. Big challenges. That's why I am keen to see the test done acoustically because hearing better sound is our chief purpose here.

Guess I was just a bit ticked-off when you told me to sign off the thread. Dunno why.

Ben
Footnote: for a long time, I've been asking to see performance specs for DSP gizmos. How much garbage of various sorts do they add to output signals? Funny, in all other areas of sound reproduction we are intent on reading performance specs. Where are the DSP specs?
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Last edited by bentoronto; 9th July 2013 at 02:02 AM.
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Old 9th July 2013, 04:56 AM   #182
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be patient my friend. I could have turned up with the finished product 6 months later and you could have said "why didn't you put this feature in it or that feature etc". In this way I spread some of my cards on the table and get some feedback and criticisms at the same time.

Regarding DSP the specs that are given don't necessarily translate into sound quality because the DSP is just a computation engine and nothing more and nothing less. It's like saying does your Intel i7 sound better than an Intel i5 ?

Some DSP's like the one I am using do have extra audio peripherals so they may have an effect on the sound qualkity if they are used. At the end of the day, given that a DSP has enough number crunching capability the effect on the audio performance has a lot to do with the word depth or how many bits per word, single or double precision, sample rate, the actual algorithms used IIR vs FIR filters , floating point vs fixed point etc. And then you have to interface it to the outside world using ADC's and DAC's and there is a lot of controversy about which part to use. Using digital interface there are a number of different standards and each has their pros and cons such as I2S vs SPDIF etc..... And then there are some DSP vendors who can actually sell you a software plugin to give it a certain tube sound.

At the end of the day you need to work out what your objectives and then focus on that rather than what is going on under the bonnet. Measurements are a good way of sorting out the wheat from the chaff and like you said at the end of the day you want to measure what you are hearing. I have no argument with that whatsoever

regards
david
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Old 9th July 2013, 10:26 PM   #183
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Quote:
Originally Posted by Tranquility Bass View Post
...Also some other accelerometer based servo systems have a problem in that when the speaker is moved the servo tries to maintain an inertial frame of reference and attempts to force the speaker cone in and out rather violently depending on the motion of the speaker cabinet. I'm happy to say that my system does not suffer from this problem and the speaker cone is rock steady when the speaker is moved much to the disappointment of one vendor that claims that this a real problem with accelerometer based servo systems...
Hello Tranquility Bass,
Is there something unique in your implementation of accelerometer based feedback that precludes issues with the servo responding to cabinet motion? Or, are you addressing this behavior via DSP processing.
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Old 9th July 2013, 11:41 PM   #184
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Hello Tranquility Bass,
Is there something unique in your implementation of accelerometer based feedback that precludes issues with the servo responding to cabinet motion?
Tranquility Bass' sensor is moving many millimeters, a well constructed cabinet's walls should move an order of magnitude less.
He hasn't loaded the speaker in a cabinet yet, but if a cabinet is floppy enough for it's motion to be a problem for servo control, that is an issue in itself.
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Old 9th July 2013, 11:53 PM   #185
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The function of the sensor is not to just respond to large motions of the cone but also to respond to vastly smaller errors of the driver, smaller, I'd guess, than the amplitude of foot-steps on a typical wood floor.

Whether the foot-steps or the counter-action of the cone are audible and/or deemed noxious is another matter. But the system has to be a stable feedback network in any case.

Ben
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Old 10th July 2013, 10:23 AM   #186
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Originally Posted by bolserst View Post
Hello Tranquility Bass,
Is there something unique in your implementation of accelerometer based feedback that precludes issues with the servo responding to cabinet motion? Or, are you addressing this behavior via DSP processing.
A combination of both

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david
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Old 10th July 2013, 10:24 AM   #187
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Originally Posted by weltersys View Post
Tranquility Bass' sensor is moving many millimeters, a well constructed cabinet's walls should move an order of magnitude less.
He hasn't loaded the speaker in a cabinet yet, but if a cabinet is floppy enough for it's motion to be a problem for servo control, that is an issue in itself.
motional feedback applied to a driver won't fix up a poorly constructed cabinet. The same rules still apply to cabinet construction

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Old 10th July 2013, 04:23 PM   #188
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A combination of both
Well, that clears things up(haha).
Care to elaborate on what about your physical/electronic implementation method addresses this concern?
DSP fixes are certainly possible, but don't interest me as much.


weltersys & bentoronto already hit on both major points.

1) Feedback loop responding to physical motion of the cabinet unrelated to the input signal.
In particular, a sharp bump from a foot can impart frequency content into the feedback loop well outside the frequency range that the input signal is filtered for. Instabilities can be provoked from bumping the cabinet even if the system appears to be perfectly stable when playing music. This is not generally a problem as most of us aren’t in the habit of kicking our subwoofers, but it can happen. In fact I was recently reminded of this fact during a visit to a local hifi shop. Rapping knuckles on the cabinet of a big name MF subwoofer to try and determine if any internal bracing was used, and I was surprised to see the woofer start oscillating at very low frequency(<20Hz)…amplitude varied reaching max of about 15mmPP and eventually died away after about 10 seconds.

2) Feedback loop responding to physical motion of cone relative to an inertial frame, where as the sound pressure that we want linearized is proportional to the motion of the cone relative to the box. In the case of a very light cone and a very heavy box, they are essentially the same. But, anybody who has experimented with large, heavy, long-throw woofers knows that there is a whole lot of box shaking going on. Once you get a woofer in a box and measure with a microphone, you will find that the odd harmonics will not be reduced as much as the accelerometer measurement suggests. I guess if a down firing woofer is used there would probably be more of an effect on even harmonics, but I did not experiment with that. In any case, the heavier the woofer and the lighter the box, the greater the disparity. Of course distortion will still be much lower than without feedback.

One simple solution for this problem is two use two woofers on opposing sides of the box such that the voice coil forces on the woofer chassis, which are bolted to the box, cancel out. It worked quite well when I tested it some years ago.

Last edited by bolserst; 10th July 2013 at 04:31 PM. Reason: added comment about down firing configuration
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Old 10th July 2013, 04:38 PM   #189
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Very clear. Thanks. I suspect all boxes shake, except maybe a Klipschorn*.

But remember, we aren't talking about big, obvious, shakes you can feel. We are talking about reducing woofer distortion from say, 2% to .5% and reducing overhang. So the dimensions of interest are quite tiny. They are .5% of the normal cone motion, which itself is usually pretty tiny.

When an accelerometer sensor budges falsely by those tiny amounts, it sends a "correction" feedback to the cone... which produces distortion. Unlike bridge and VC feedback, an accelerometer is not keyed to the motion of the cone relative to the driver basket, but to the outside cosmos. Although not a feasible sensor method anyway, box movement might also be a deficiency of mic sensing.

....unless, heaven forbid, your music room has a concrete floor (and your speaker box is screwed firmly to it), in which case you might be OK.

I believe that feedback based on bridge or second voice coil sensing is pretty much immune to reacting to these external forces and indeed might just correct a wayward cone from budging at all (at its resonance) after rapping real hard on the back of the box.

Ben
*I keep my wine rack on top ot the Klipschorn anyway - very convenient. At other times, with my wife's approval, I have put cinderblocks on speakers. Some people use spike-like feet. Does Monster sell special HiFi cinderblocks?
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Last edited by bentoronto; 10th July 2013 at 04:48 PM.
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Old 10th July 2013, 05:03 PM   #190
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Quote:
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...I believe that feedback based on bridge or second voice coil sensing is pretty much immune to reacting to these external forces ...
This is true, voice coil feedback signal is proportional to relative motion between woofer and the box; just what we want. However feedback from a voice coil has its own linearity issues from BL variation with position, which is one of the main sources of woofer distortion in the first place. The voice coil feedback signal is in phase with the velocity of the cone, which is NOT in phase with the position. So correction of the BL-vs-position variation isn't a particularly simple task, although I'm sure DSP could handle it.

I could be wrong, but I think the voice coil MF systems available today tend to be less prone to reacting to external forces mainly because they use much lower loop gain.
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